From the studies on 2010.08.31, and again in the addendum from 2011.01.12, we have seen residual amplitude-dependent tuneshift when all sextupoles have been disabled in the lattice. Here, fits have been provided for the three scenarios previously discussed (2fam sextupoles, optimized sextupoles, and sextupoles off).
To compute alphas, second-order curve fits are performed (DQ = A * y**2). To convert to alpha, recall y**2 = 2 \beta J. Amplitudes are set at the center of L0, therefore beta.x = 7.08m, beta.y = 10.59m for the CHESS low-emittance optics.
Recall: BMADz believes the sextupole contribution to alpha_yy is 193/m (using Bengtsson) or -637/m (using S.Y. Lee) for the 2-family distribution. From tracking, the total vertical tuneshift with 2fam sextupoles is 520.1/m, and when sextupoles are disabled, vertical tuneshift is 269/m; the difference (i.e., the contribution from sextupoles) is 251/m.
Update: 2011.01.30 - Turns out sex_47e/w were not being set to zero when running the "sextupoles off" scenario. See 2011.01.28 update for corrected results.
